Through vapor pressure measurements, water activities of the glycerol water solution were first examined under temperatures of 30-85 degrees C and glycerol concentrations of 0-50 mol %. The water activity is the only determinant of phase-transition point among CaSO4 center dot 2H(2)O (DH), alpha-CaSO4 center dot 0.5H(2)O (alpha-HH), and CaSO4 (AR) in glycerol water solution. Accordingly, a phase-transition diagram was constructed as a function of temperature (0-140 degrees C) and glycerol concentration (0-50 mol %), which provided a thermodynamic preparation window of alpha-HH from DH. Controlling glycerol concentration and temperature within the window, direct transition of DH to alpha-HH can be regulated. However, the window is apt to shrink as the operating time is shortened, indicating that its available profile is directed by transition kinetics of DH to alpha-HH besides transition thermodynamics. Overall, the glycerol water solution could act as a novel category of nonelectrolyte medium for alpha-HH preparation from DH.